1. Field of the Invention
This invention relates to an optical information reproducing apparatus, and more particularly to a track accessing circuit which can perform steady and exact track access functions in connection with an optical information reproducing operation. 2. Description of the Related Art
In general, an optical information reproducing apparatus uses an optical pick-up to reproduce optical information from a recording medium. A recording medium, which is called an optical disc, has a plurality of sectors each extending radially thereof and tracks provided concentrically thereon. A number of pits which represent information to be recorded, are formed on each track. The length of the pits differs according to what information is recorded. The operation of reading out or reproducing information recorded is effected as follows:
First, a laser beam is applied to a preset track on a recording medium via an optical pick-up having an objective lens. The laser beam is reflected toward the objective lens and the amount of the beam reflected varies depending on the presence or absence of the pit. When the laser beam is applied to a position having no pit, an intense light goes back toward the objective lens. In contrast, when the laser beam hits the pit, the beam diffracts and a portion of the light reflected is not transmitted toward the objective lens, thus reducing the amount of the reflected light incident on the objective lens. Therefore, information can be divided from detecting the difference in the amount of light reflected.
To detect the difference, a light sensing device is used.
In order to precisely reproduce a signal by applying a laser beam to the optical disc, it is necessary to control the optical pick-up or objective lens so that the laser beam can always be applied to a preset pit position in a preset track. That is, when the laser beam spot is deviated from the preset pit position because of the eccentricity of the disc or distortion of the track, a servo system operates to return the optical pick-up to the preset pit position. In general, the above control operation is called tracking servo control. As the tracking servo control, a three spots method a wobbling method, a push-pull method and the like are known.
Japanese patent publication No. 58-55566 discloses a tracking servo control in which an electrical feedback circuit is provided for controlling a motor device used for shifting a lens, the amount of the shift depending on a voltage supplied from a photoelectric converter which converts light reflected from a track into electrical signals.
When a signal is reproduced from the disc, it is necessary for the optical pick-up to access the preset pit position of the preset track in order to readout information recorded on the disc. This is called track access. The track access method includes the following three methods. The first method is t move the entire optical pick-up so as to access a preset track position; the second method is to move only the objective lens without moving the optical pick-up so as to jump a plurality of tracks at one time: and the third method is to move the objective lens so as to jump the track one by one and access a preset track position.
The first method is used for access of above 50 gm (corresponding to 30 tracks). However, the first method provides imprecise access and takes a long access time. Further, in the third method, the access precision is high but it takes the longest access time among the three methods.
Now, a case is explained in which the objective lens jumps 30 tracks or above by utilizing the second method.
In this case, since the objective lens travels over a distance of 30 tracks or above, the deviation of the objective lens from the optical pick-up becomes large. Since both ends of the objective lens are supported by a pair of leaf springs respectively one of the leaf springs is compressed and the other is expanded after the movement of the objective lens. In this case, the objective lens tend to go back to the original position by restoring force of the leaf springs, creating deviation from the position accessed by the objective lens.
In order to solve the above problem, a laser beam spot will not be set exactly on the pit but can be slightly deviated from the pit or only partly set on the pit. From this, a track-error signal is generated, and is supplied to a driving coil for driving the objective lens oppositely from its restoring direction. Thus, this signal is used for preventing the objective lens from being deviated from the accessed position.
However, in this method, time required for the track-error signal to be supplied to the driving coil can often become longer than that for the leaf springs to be restored. In such a case, the objective lens can be deviated from the accessed position by the restoring force of the leaf springs before the driving coil is activated by the track-error signal to maintain the objective lens at a position accessed. Therefore, it has long been required to develop a track accessing circuit having stable functions and capable of preventing the objective lens from being deviated from the accessed track position even in the case of performing access operation over 30 tracks or above.